Major advances have been achieved during the last year in respect to the genetics of the disorders of peroxisome biogenesis. Eleven complementation groups, each of which are presumed to represent distinct genotypes, have been identified, and their phenotypes have been characterized. For two of the disorders the gene defect and mutations have been defined. Therapeutic approaches involve the administration nof a microencapsulated omega 3 fatty acid, Docosahexaeonic (DHA). DHA has an important role in retina and brain, and is severely deficient in patients with the Zellweger syndrome and neonatal adrenoleukodystrophy. Our studies of 13 patients show that the present method of providing DHA is easily accomplished, even in patients with severe disability, and that it leads to rapid normalization of DHA levels in plasma and in red blood cells. A double-blinded placebo controlled study of the effect of DHA administration on the clinical course of theese disorders is now in progress.